Direct HIV cytopathicity cannot account for CD4 decline in AIDS in the presence of homeostasis: a worst-case dynamic analysis

Bifurcation and stability analysis of within host HIV dynamics with multiple infections and intracellular delay

Human immunodeficiency virus (HIV) manifests multiple infections in CD4+ T cells, by binding its envelope proteins to CD4 receptors. Understanding these biological processes is crucial for effective interventions against HIV/AIDS. Here, we propose a mathematical model that accounts for the multiple infections of CD4+ T cells and an intracellular delay in the dynamics of HIV infection. We study the model system and establish the conditions under which the disease-free equilibrium point and the endemic equilibrium point are locally and globally asymptotically stable. We further provide the conditions under which these equilibrium points undergo forward or backward transcritical bifurcations for the autonomous model and Hopf bifurcation for both the delay model and autonomous models. Our simulation results show that an increase in the rate of multiple infections of CD4+ T cells stabilizes the endemic equilibrium point through Hopf bifurcation. However, in the presence of an intracellular delay, the model system evinces three types of stability scenarios at the endemic equilibrium point-instability switch, stability switch, and stability invariance and is demonstrated using bi-parameter diagrams. One of the novel aspects of this study is exhibiting all these interesting nonlinear dynamical results within a single model incorporating a single time delay.

Mathematical Models of HIV-1 Dynamics, Transcription, and Latency

HIV-1 latency is a major barrier to curing infections with antiretroviral therapy and, consequently, to eliminating the disease globally. The establishment, maintenance, and potential clearance of latent infection are complex dynamic processes and can be best described with the help of mathematical models followed by experimental validation. Here, we review the use of viral dynamics models for HIV-1, with a focus on applications to the latent reservoir. Such models have been used to explain the multi-phasic decay of viral load during antiretroviral therapy, the early seeding of the latent reservoir during acute infection and the limited inflow during treatment, the dynamics of viral blips, and the phenomenon of post-treatment control. Finally, we discuss that mathematical models have been used to predict the efficacy of potential HIV-1 cure strategies, such as latency-reversing agents, early treatment initiation, or gene therapies, and to provide guidance for designing trials of these novel interventions.

Immunological Paradigms, Mechanisms, and Models: Conceptual Understanding Is a Prerequisite to Effective Modeling

Most mathematical models that describe the individual or collective actions of cells aim at creating faithful representations of limited sets of data in a self-consistent manner. Consistency with relevant physiological rules pertaining to the greater picture is rarely imposed. By themselves, such models have limited predictive or even explanatory value, contrary to standard claims. Here I try to show that a more critical examination of currently held paradigms is necessary and could potentially lead to models that pass the test of time. In considering the evolution of paradigms over the past decades I focus on the “smart surveillance” theory of how T cells can respond differentially, individually and collectively, to both self- and foreign antigens depending on various “contextual” parameters. The overall perspective is that physiological messages to cells are encoded not only in the biochemical connections of signaling molecules to the cellular machinery but also in the magnitude, kinetics, and in the time- and space-contingencies, of sets of stimuli. By rationalizing the feasibility of subthreshold interactions, the “dynamic tuning hypothesis,” a central component of the theory, set the ground for further theoretical and experimental explorations of dynamically regulated immune tolerance, homeostasis and diversity, and of the notion that lymphocytes participate in nonclassical physiological functions. Some of these efforts are reviewed. Another focus of this review is the concomitant regulation of immune activation and homeostasis through the operation of a feedback mechanism controlling the balance between renewal and differentiation of activated cells. Different perspectives on the nature and regulation of chronic immune activation in HIV infection have led to conflicting models of HIV pathogenesis—a major area of research for theoretical immunologists over almost three decades—and can have profound impact on ongoing HIV cure strategies. Altogether, this critical review is intended to constructively influence the outlook of prospective model builders and of interested immunologists on the state of the art and to encourage conceptual work.

Insight into treatment of HIV infection from viral dynamics models

The odds of living a long and healthy life with HIV infection have dramatically improved with the advent of combination antiretroviral therapy. Along with the early development and clinical trials of these drugs, and new field of research emerged called viral dynamics, which uses mathematical models to interpret and predict the time-course of viral levels during infection and how they are altered by treatment. In this review, we summarize the contributions that virus dynamics models have made to understanding the pathophysiology of infection and to designing effective therapies. This includes studies of the multiphasic decay of viral load when antiretroviral therapy is given, the evolution of drug resistance, the long-term persistence latently infected cells, and the rebound of viremia when drugs are stopped. We additionally discuss new work applying viral dynamics models to new classes of investigational treatment for HIV, including latency-reversing agents and immunotherapy.

Mathematical Models of HIV Latency

Viral latency is a major barrier to curing HIV infection with antiretroviral therapy, and consequently, for eliminating the disease globally. The establishment, maintenance, and potential clearance of latent infection are complex dynamic processes and can be best understood and described with the help of mathematical models. Here we review the use of viral dynamics models for HIV, with a focus on applications to the latent reservoir. Such models have been used to explain the multiphasic decay of viral load during antiretroviral therapy, the early seeding of the latent reservoir during acute infection and the limited inflow during treatment, the dynamics of viral blips, and the phenomenon of posttreatment control. In addition, mathematical models have been used to predict the efficacy of potential HIV cure strategies, such as latency-reversing agents, early treatment initiation, or gene therapies, and to provide guidance for designing trials of these novel interventions.

A transcriptome-based model of central memory CD4 T cell death in HIV infection

BACKGROUND

Human central memory CD4 T cells are characterized by their capacity of proliferation and differentiation into effector memory CD4 T cells. Homeostasis of central memory CD4 T cells is considered a key factor sustaining the asymptomatic stage of Human Immunodeficiency Virus type 1 (HIV-1) infection, while progression to acquired immunodeficiency syndrome is imputed to central memory CD4 T cells homeostatic failure. We investigated if central memory CD4 T cells from patients with HIV-1 infection have a gene expression profile impeding proliferation and survival, despite their activated state.

METHODS

Using gene expression microarrays, we analyzed mRNA expression patterns in naive, central memory, and effector memory CD4 T cells from healthy controls, and naive and central memory CD4 T cells from patients with HIV-1 infection. Differentially expressed genes, defined by Log₂ Fold Change (FC) ≥ |0.5| and Log (odds) > 0, were used in pathway enrichment analyses.

RESULTS

Central memory CD4 T cells from patients and controls showed comparable expression of differentiation-related genes, ruling out an effector-like differentiation of central memory CD4 T cells in HIV infection. However, 210 genes were differentially expressed in central memory CD4 T cells from patients compared with those from controls. Expression of 75 of these genes was validated by semi quantitative RT-PCR, and independently reproduced enrichment results from this gene expression signature. The results of functional enrichment analysis indicated movement to cell cycle phases G1 and S (increased CCNE1, MKI67, IL12RB2, ADAM9, decreased FGF9, etc.), but also arrest in G2/M (increased CHK1, RBBP8, KIF11, etc.). Unexpectedly, the results also suggested decreased apoptosis (increased CSTA, NFKBIA, decreased RNASEL, etc.). Results also suggested increased IL-1β, IFN-γ, TNF, and RANTES (CCR5) activity upstream of the central memory CD4 T cells signature, consistent with the demonstrated milieu in HIV infection.

CONCLUSIONS

Our findings support a model where progressive loss of central memory CD4 T cells in chronic HIV-1 infection is driven by increased cell cycle entry followed by mitotic arrest, leading to a non-apoptotic death pathway without actual proliferation, possibly contributing to increased turnover.

High levels of anti-Nef antibodies may prevent AIDS disease progression in vertically HIV-1-infected infants

Introduction

HIV-1-associated CD4+ T-cell depletion is a consequence of uninfected cell death. Nef is one of the viral factors that trigger apoptosis on bystander cells, though the plasma Nef levels do not correlate with Th lymphocytes counts. The aim of our study was to evaluate whether anti-Nef antibodies were involved in paediatric AIDS development and whether they can prevent the CD4+ T-cell depletion in vertically infected children.

Methods

Two hundred and seventy three HIV-1 vertically infected children seen at Garrahan Paediatric Hospital were randomly included in the study, adding 13 selected cases: seven LTNP (long-term non-progressors) and six RP (rapid progressors) children (n_total=286). Specific anti-HIV-1-Nef antibodies were titrated by indirect ELISA and compared between groups. The plasma blocking effect on Nef-dependent cytotoxicity was evaluated in Jurkat cells using recombinant Nef as apoptotic stimulus and patient plasmas as blockers, measuring the apoptotic levels using Annexin-V stain and flow cytometry.

Results

Only 63.4% of the patients had specific anti-Nef antibodies, and the levels of anti-Nef antibodies found in the selected LTNPs plasmas were always significantly higher (p=1.55×10⁻⁴) than those in RPs or general HIV-1+ paediatric populations. The LTNPs’ plasma had a strong inhibitory effect on Nef-dependent cytotoxicity even at high dilutions, while RP plasmas had little or no effect on Nef-induced apoptosis.

Discussion and conclusions

High anti-Nef antibody levels are associated and predict slow or non-progression to AIDS in vertically HIV-1-infected children. They could be an efficient tool in preventing Nef-associated bystander effect, preserving CD4+ T-cells and the immune function in the context of paediatric HIV-1 infection.

Antigen-dependent and -independent mechanisms of T and B cell hyperactivation during chronic HIV-1 infection

Continuous loss of CD4(+) T lymphocytes and systemic immune activation are hallmarks of untreated chronic HIV-1 infection. Chronic immune activation during HIV-1 infection is characterized by increased expression of activation markers on T cells, elevated levels of proinflammatory cytokines, and B cell hyperactivation together with hypergammaglobulinemia. Importantly, hyperactivation of T cells is one of the best predictive markers for progression toward AIDS, and it is closely linked to CD4(+) T cell depletion and sustained viral replication. Aberrant activation of T cells is observed mainly for memory CD4(+) and CD8(+) T cells and is documented, in addition to increased expression of surface activation markers, by increased cell cycling and apoptosis. Notably, the majority of these activated T cells are neither HIV specific nor HIV infected, and the antigen specificities of hyperactivated T cells are largely unknown, as are the exact mechanisms driving their activation. B cells are also severely affected by HIV-1 infection, which is manifested by major changes in B cell subpopulations, B cell hyperactivation, and hypergammaglobulinemia. Similar to those of T cells, the mechanisms underlying this aberrant B cell activation remain largely unknown. In this review, we summarized current knowledge about proposed antigen-dependent and -independent mechanisms leading to lymphocyte hyperactivation in the context of HIV-1 infection.

Aging and HIV infection

INTRODUCTION

Population aging has become a global phenomenon, and HIV infection among older individuals is also increasing. Because age can affect the progression of HIV infection, we aimed to evaluate the present knowledge on HIV infection in older patients.

METHODS

Literature review of the last 20 years.

RESULTS

Older HIV-infected patients have lower CD4(+) T cell counts, higher viral load and are more frequently symptomatic at diagnosis. The infection progresses more rapidly, with higher morbidity and lethality rates. However, older patients are more compliant to antiretroviral treatment; they experience a better virologic response, and treatment represents a positive clinical impact. Aging affects the complex interaction between HIV infection and the immune system. Both conditions contribute to the dysfunction of immune cells, including a decrease in the phagocytes' microbicidal capability, natural killer cells' cytolytic function, expression of toll-like receptors and production of interleukin-12. Chronic immune activation responsible for the depletion of CD4(+) and CD8(+) T cells in HIV infection appears to worsen with senescence. Older patients also exhibit a less robust humoral response, with the production of less avid and specific antibodies.

CONCLUSION

Both HIV and aging contribute to immune dysfunction, morbidity and mortality. However, highly active antiretroviral therapy (HAART) is beneficial for older patients, and treatment of older patients should not be discouraged.

Lineage-specific T-cell reconstitution following in vivo CD4+ and CD8+ lymphocyte depletion in nonhuman primates

Many features of T-cell homeostasis in primates are still unclear, thus limiting our understanding of AIDS pathogenesis, in which T-cell homeostasis is lost. Here, we performed experiments of in vivo CD4(+) or CD8(+) lymphocyte depletion in 2 nonhuman primate species, rhesus macaques (RMs) and sooty mangabeys (SMs). Whereas RMs develop AIDS after infection with simian immunodeficiency virus (SIV), SIV-infected SMs are typically AIDS-resistant. We found that, in both species, most CD4(+) or CD8(+) T cells in blood and lymph nodes were depleted after treatment with their respective antibodies. These CD4(+) and CD8(+) lymphocyte depletions were followed by a largely lineage-specific CD4(+) and CD8(+) T-cell proliferation, involving mainly memory T cells, which correlated with interleukin-7 plasma levels. Interestingly, SMs showed a faster repopulation of naive CD4(+) T cells than RMs. In addition, in both species CD8(+) T-cell repopulation was faster than that of CD4(+) T cells, with CD8(+) T cells reconstituting a normal pool within 60 days and CD4(+) T cells remaining below baseline levels up to day 180 after depletion. While this study revealed subtle differences in CD4(+) T-cell repopulation in an AIDS-sensitive versus an AIDS-resistant species, such differences may have particular relevance in the presence of active SIV repli cation, where CD4(+) T-cell destruction is chronic.

Pathogenic mechanisms in simian immunodeficiency virus infection

PURPOSE OF REVIEW

Recent work in pathogenic simian immunodeficiency (SIV) infection of Asian macaques and in natural, nonpathogenic SIV infections of African nonhuman primate species has demonstrated that persistent activation has profound effects on CD4+ memory T-cell proliferation, differentiation and survival. Disease progression in pathogenic infection has been closely linked to these dynamics, reflecting a complex interplay of virus-mediated killing, the effects of systemic activation and host regenerative mechanisms. We review these recent advances.

RECENT FINDINGS

Massive depletion of CD4+ effector-memory T cells invariably occurs during acute CCR5-tropic SIV infection, but is initially stabilized by new production of these cells from spared central memory precursors above the threshold required to maintain clinical immune competence. In pathogenic (but not natural, apathogenic) infections, a persistent state of immune activation, characterized by multiple, recurrent bursts of lymphocyte proliferation, differentiation, migration, death and functional modification of 'resting' cells, is associated with progressive depletion of central memory CD4+ T cells, and ultimately, a collapse of effector site CD4+ memory populations that is closely associated with overt immune deficiency.

SUMMARY

The importance of maintaining the regenerative capacity of the central-memory compartment of CD4+ T cells is increasingly evident. Defining the physiologic and molecular mechanisms responsible for instability of the CD4+ central-memory T cell pool could enable new immunotherapeutic interventions.

Mathematical modeling of viral kinetics under immune control during primary HIV-1 infection

Primary human immunodeficiency virus (HIV) infection is characterized by an initial exponential increase of viral load in peripheral blood reaching a peak, followed by a rapid decline to the viral setpoint. Although the target-cell-limited model can account for part of the viral kinetics observed early in infection [Phillips, 1996. Reduction of HIV concentration during acute infection: independence from a specific immune response. Science 271 (5248), 497-499], it frequently predicts highly oscillatory kinetics after peak viremia, which is not typically observed in clinical data. Furthermore, the target-cell-limited model is unable to predict long-term viral kinetics, unless a delayed immune effect is assumed [Stafford et al., 2000. Modeling plasma virus concentration during primary HIV infection. J. Theor. Biol. 203 (3), 285-301]. We show here that extending the target-cell-limited model, by implementing a saturation term for HIV-infected cell loss dependent upon infected cell levels, is able to reproduce the diverse observed viral kinetic patterns without the assumption of a delayed immune response. Our results suggest that the immune response may have significant effect on the control of the virus during primary infection and may support experimental observations that an anti-HIV immune response is already functional during peak viremia.

The Distribution and Immune Profile of T Cell Subsets in HIV-Infected Children from Uganda

Abstract T cell activation is an important mechanism in HIV-associated immune depletion. We have previously demonstrated an association between the hyperactivation of CD4(+) and CD8(+) T cells and low CD4 status in HIV-infected Ugandan children. In this study, we explore differences in activation between naive and memory T cells in HIV-infected Ugandan children. A significant correlation between CD4- and CD8-mediated immune activation and CD4 status was observed only in the memory T cells. Antiretroviral (ART) untreated and treated HIV-positive and HIV-negative children displayed similar profiles of activation and distribution within the CD4(+) naive T cells. In contrast, significantly higher immune activation of the memory CD4(+) T cell subset was seen in ART-untreated children when compared to ART-treated or HIV-negative children. ART-mediated viral suppression led to the correction of CD4(+) immune activation to levels seen in uninfected children but did not increase the size of the memory CD4(+) T cell population. High levels of CD8(+) immune activation were also found in both naive and memory cell subsets. Antiretroviral treatment led to the normalization of CD8(+) T cell activation but did not correct the distribution of naive CD8(+) T cells. We also assessed PD-1 expression on CD8(+) T cells as a measure of immune dysfunction. Upregulation of PD-1 was highest in untreated children but persisted in ART-treated children compared to uninfected children. The mechanisms of immunopathogenesis in pediatric HIV infection likely involve distinct contributions from individual naive and memory T cells subsets.

Prevention of immunodeficiency virus induced CD4+ T-cell depletion by prior infection with a non-pathogenic virus

Immune dysregulation initiated by a profound loss of CD4+ T-cells is fundamental to HIV-induced pathogenesis. Infection of domestic cats with a non-pathogenic lentivirus prevalent in the puma (puma lentivirus, PLV or FIV(pco)) prevented peripheral blood CD4+ T-cell depletion caused by subsequent virulent FIV infection. Maintenance of this critical population was not associated with a significant decrease in FIV viremia, lending support to the hypothesis that direct viral cytopathic effect is not the primary cause of immunodeficiency. Although this approach was analogous to immunization with a modified live vaccine, correlates of immunity such as a serum-neutralizing antibody or virus-specific T-cell proliferative response were not found in protected animals. Differences in cytokine transcription profile, most notably in interferon gamma, were observed between the protected and unprotected groups. These data provide support for the importance of non-adaptive enhancement of the immune response in the prevention of CD4+ T-cell loss.

Chronic CD4+ T-cell activation and depletion in human immunodeficiency virus type 1 infection: type I interferon-mediated disruption of T-cell dynamics

The mechanism of CD4(+) T-cell depletion during chronic human immunodeficiency virus type 1 (HIV-1) infection remains unknown. Many studies suggest a significant role for chronic CD4(+) T-cell activation. We assumed that the pathogenic process of excessive CD4(+) T-cell activation would be reflected in the transcriptional profiles of activated CD4(+) T cells. Here we demonstrate that the transcriptional programs of in vivo-activated CD4(+) T cells from untreated HIV-positive (HIV(+)) individuals are clearly different from those of activated CD4(+) T cells from HIV-negative (HIV(-)) individuals. We observed a dramatic up-regulation of cell cycle-associated and interferon-stimulated transcripts in activated CD4(+) T cells of untreated HIV(+) individuals. Furthermore, we find an enrichment of proliferative and type I interferon-responsive transcription factor binding sites in the promoters of genes that are differentially expressed in activated CD4(+) T cells of untreated HIV(+) individuals compared to those of HIV(-) individuals. We confirm these findings by examination of in vivo-activated CD4(+) T cells. Taken together, these results suggest that activated CD4(+) T cells from untreated HIV(+) individuals are in a hyperproliferative state that is modulated by type I interferons. From these results, we propose a new model for CD4(+) T-cell depletion during chronic HIV-1 infection.

Aflatoxin-related immune dysfunction in health and in human immunodeficiency virus disease

Both aflatoxin and the human immunodeficiency virus (HIV) cause immune suppression and millions of HIV-infected people in developing countries are chronically exposed to aflatoxin in their diets. We investigated the possible interaction of aflatoxin and HIV on immune suppression by comparing immune parameters in 116 HIV positive and 80 aged-matched HIV negative Ghanaians with high (> or =0.91 pmol/mg albumin) and low (<0.91 pmol/mg albumin) aflatoxin B1 albumin adduct (AF-ALB) levels. AF-ALB levels and HIV viral load were measured in plasma and the percentages of leukocyte immunophenotypes and cytokine expression were determined using flow cytometry. The cross-sectional comparisons found that (1) among both HIV positive and negative participants, high AF-ALB was associated with lower perforin expression on CD8+ T-cells (P = .012); (2) HIV positive participants with high AF-ALB had significantly lower percentages of CD4+ T regulatory cells (Tregs; P = .009) and naive CD4+ T cells (P = .029) compared to HIV positive participants with low AF-ALB; and (3) HIV positive participants with high AF-ALB had a significantly reduced percentage of B-cells (P = .03) compared to those with low AF-ALB. High AF-ALB appeared to accentuate some HIV associated changes in T-cell phenotypes and in B-cells in HIV positive participants.

Understanding the slow depletion of memory CD4+ T cells in HIV infection

BACKGROUND

The asymptomatic phase of HIV infection is characterised by a slow decline of peripheral blood CD4(+) T cells. Why this decline is slow is not understood. One potential explanation is that the low average rate of homeostatic proliferation or immune activation dictates the pace of a "runaway" decline of memory CD4(+) T cells, in which activation drives infection, higher viral loads, more recruitment of cells into an activated state, and further infection events. We explore this hypothesis using mathematical models.

METHODS AND FINDINGS

Using simple mathematical models of the dynamics of T cell homeostasis and proliferation, we find that this mechanism fails to explain the time scale of CD4(+) memory T cell loss. Instead it predicts the rapid attainment of a stable set point, so other mechanisms must be invoked to explain the slow decline in CD4(+) cells.

CONCLUSIONS

A runaway cycle in which elevated CD4(+) T cell activation and proliferation drive HIV production and vice versa cannot explain the pace of depletion during chronic HIV infection. We summarize some alternative mechanisms by which the CD4(+) memory T cell homeostatic set point might slowly diminish. While none are mutually exclusive, the phenomenon of viral rebound, in which interruption of antiretroviral therapy causes a rapid return to pretreatment viral load and T cell counts, supports the model of virus adaptation as a major force driving depletion.

Biophysical characterization of V3-lipopeptide liposomes influencing HIV-1 infectivity

The V3-loop of the HIV-1 gp120 alters host cell immune function and modulates infectivity. We investigated biophysical parameters of liposome constructs with embedded lipopeptides from the principle neutralizing domain of the V3-loop and their influence on viral infectivity. Dynamic light scattering measurements showed liposome supramolecular structures with hydrodynamic radius of the order of 900 and 1300nm for plain and V3-lipopeptide liposomes. Electron paramagnetic resonance measurements showed almost identical local microenvironment. The difference in liposome hydrodynamic radius was attributed to the fluctuating ionic environment of the V3-lipopeptide liposomes. In vitro HIV-1 infectivity assays showed that plain liposomes reduced virus production in all cell cultures, probably due to the hydrophobic nature of the aggregates. Liposomes carrying V3-lipopeptides with different cationic potentials restored and even enhanced infectivity (p<0.05). These results highlight the need for elucidation of the involvement of lipid bilayers as dynamic components in supramolecular structures and in HIV-1 fusion mechanisms.

Identifying the membrane proteome of HIV-1 latently infected cells

Profiling integral plasma membrane proteins is of particular importance for the identification of new biomarkers for diagnosis and for drug development. We report in this study the identification of surface markers by performing comparative proteomics of established human immunodeficiency virus-1 (HIV-1) latent cell models and parental cell lines. To this end we isolated integral membrane proteins using a biotin-directed affinity purification method. Isolated proteins were separated by two-dimensional gel electrophoresis and identified by matrix-assisted laser desorption/ionization-time-of-flight (MALDI-TOF) after in gel digestion. Seventeen different proteins were found to vary on the surface of T-cells due to HIV-1 infection. Of these proteins, 47% were integral membrane proteins, and 18% were membrane-associated. Through the use of complementary techniques such as Western blotting and fluorescent staining, we confirmed the differential expression of some of the proteins identified by MALDI-TOF including Bruton's tyrosine kinase and X-linked inhibitor of apoptosis. Finally, using phosphatidylinositol 3-kinase inhibitors and flavopiridol to inhibit Bruton's tyrosine kinase localization at the membrane and X-linked inhibitor of apoptosis protein expression, respectively, we showed that HIV-1 latently infected cells are more sensitive to these drugs than uninfected cells. This suggests that HIV-1 latently infected cells may be targeted with drugs that alter several pathways that are essential for the establishment and maintenance of latency.

In HIV-1 pathogenesis the die is cast during primary infection

The chronic stage of HIV-1 infection has been extensively described as a slowly evolving phase, in which the virus induces T-cell death slightly faster than the human body is able to recover. In contrast, T-cell and viral replication dynamics during primary infection have been less well studied. Recent studies in the SIV-macaque model and in HIV-positive patients during the acute infection period have highlighted the massive and irreversible depletion of CD4 memory T cells in the mucosa, particularly in the gut. Hence, gut-associated lymphoid tissue (GALT) plays a central role in the early stages of HIV-1 pathogenesis. Due to its particular cytokine expression pattern, GALT may favour the differential replication of certain HIV-1 subtypes during primary infection, particularly of subtype C. This could enhance the chance of a successful transmission. Moreover, these early events taking place in GALT during primary infection have major implications for therapy and vaccine design.

Pathogenesis of HIV infection: what the virus spares is as important as what it destroys

Upon transmission to a new host, HIV targets CCR5+ CD4+ effector memory T cells, resulting in acute, massive depletion of these cells from mucosal effector sites. This depletion does not initially compromise the regenerative capacity of the immune system because naive and most central memory T cells are spared. Here, we discuss evidence suggesting that frequent activation of these spared cells during the chronic phase of HIV infection supplies mucosal tissues with short-lived CCR5+ CD4+ effector cells that prevent life-threatening infections. This immune activation also facilitates continued viral replication, but infection and killing of target T cells by HIV are selective and the impact on effector-cell lifespan is limited. We propose, however, that persistent activation progressively disrupts the functional organization of the immune system, reducing its regenerative capacity and facilitating viral evolution that leads to loss of the exquisite target cell-sparing selectivity of viral replication, ultimately resulting in AIDS.

Density-dependent decay in HIV-1 dynamics

The decay of HIV-1-infected cell populations after treatment with antiretroviral therapy has been measured using simple exponential decay models. These models are unlikely to be realistic over periods longer than a few months, however, because the population dynamics of HIV are complex. We considered an alternate model developed by Perelson and colleagues that extends the standard model for biphasic viral load decline and allows for nonlinear log decay of infected cell populations. Using data from 6 children on highly active antiretroviral therapy (HAART) and a single parameter in the new model, the assumption of log linear decay of infected cell populations is tested. Our analysis indicates that the short-lived and long-lived infected cell populations do not decay according to a simple exponential model. Furthermore, the resulting estimates of time to eradication of infected cell compartments are dramatically longer than those previously reported (eg, decades vs. years for long-lived infected cell populations and years vs. weeks for short-lived infected cell populations). Furthermore, estimates of the second-phase decay rates are significantly different than 0 for most children when obtained using the Perelson biphasic decay model. In contrast, this rate is not significantly different than 0 when the density-dependent decay model is used for parameter estimation and inference. Thus, the density-dependent decay model but not the simple exponential decay model is consistent with recent data showing that even under consistent HAART-mediated suppression of viral replication, decay rates of infected cell reservoirs decay little over several years. This suggests that conclusions about long-term viral dynamics of HIV infection based on simple exponential decay models should be carefully re-evaluated.

Correlation of T-lymphocyte subpopulations with immunological markers in HIV-1-infected Indian patients

Progressive HIV disease is characterized by CD4 T cell decline and activation of the immune system. We aimed to study the quantitative alterations in the naive (CD45RA+CD62L+), memory/effector (CD45RO+) and activated (HLA-DR+CD38+) T-lymphocyte subpopulations in antiretroviral treatment naive, HIV-1 infected Indian patients by three-color multi-parametric flow cytometry. The association of different CD4+ and CD8+ T cell subsets with the immunological markers- CD4+ and CD8+ T cell percentages was examined by calculating the partial correlation coefficients. We also observed significant differences in the expression of different CD4+ and CD8+ T-cell subsets among the two groups of patients formed using the median CD4+ T cell percentage value (15%) of the study population. The correlations of different CD4+ and CD8+ T cell subsets reflected the quantitative alterations in the T-lymphocyte subpopulations and activation of the immune system during HIV-infection. The study outcome also emphasizes the significance of the CD38+CD8+ T-lymphocyte subset as a prognostic marker for HIV management and ART monitoring in resource-limited settings of developing countries like India.

Naturally SIV-infected sooty mangabeys: are we closer to understanding why they do not develop AIDS?

Simian immunodeficiency viruses (SIV) infection of sooty mangabey (SM) monkeys (Cercocebus atys), a natural host species, does not induce CD4+ T cell depletion and acquired immunodeficiency syndrome (AIDS) despite chronic high levels of virus replication. In contrast, SIV infection of non-natural host species, such as rhesus macaques (RM), induces a disease that closely resembles AIDS in humans. The mechanisms underlying the lack of disease progression in SIV-infected SMs are incompletely understood, but certainly reflect a complex evolutionary adaptation whereby the host immune system is not significantly damaged by the highly replicating virus. It is now widely recognized that a better understanding of these mechanisms may provide clues to the pathogenesis of immunodeficiency in HIV-infected humans. In this article I discuss five different hypotheses that may account for the non-pathogenic course of infection in SIV-infected SMs and briefly review the available data supporting each of these hypotheses.

Distinct profile of T cell activation in HIV type 2 compared to HIV type 1 infection: differential mechanism for immunoprotection

The mechanism for the lower rate of disease progression in HIV-2 infection remains undefined. We evaluated T cell activation in a cohort of HIV-infected commercial sex workers in Dakar, Senegal. CD8+ T cell activation was significantly lower in HIV-2- compared to HIV-1-infected volunteers and both groups displayed higher activation levels compared to seronegative individuals. In contrast, CD4+ T cell activation was similar between the HIV-1 and HIV-2 groups and significantly higher compared to the seronegative group. Interestingly, HIV-2-positive volunteers with evidence of Gag-specific CD8+ T cell responses displayed lower CD4+ T cell activation. Our data suggest that the distinct T cell activation profile in HIV-2-positive individuals may reflect on the presence of effective host immune responses in HIV-2 infection.

Depletion of Regulatory T Cells in HIV Infection Is Associated with Immune Activation

Immune activation during chronic HIV infection is a strong clinical predictor of death and may mediate CD4(+) T cell depletion. Regulatory T cells (Tregs) are CD4(+)CD25(bright)CD62L(high) cells that actively down-regulate immune responses. We asked whether loss of Tregs during HIV infection mediates immune activation in a cross-sectional study of 81 HIV-positive Ugandan volunteers. We found that Treg number is strongly correlated with both CD4(+) and CD8(+) T cell activation. In multivariate modeling, this relationship between Treg depletion and CD4(+) T cell activation was stronger than any other clinical factor examined, including viral load and absolute CD4 count. Tregs appear to decline at different rates compared with other CD4(+) T cells, resulting in an increased regulator to helper ratio in many patients with advanced disease. We hypothesize that this skewing may contribute to T cell effector dysfunction. Our findings suggest Tregs are a major contributor to the immune activation observed during chronic HIV infection.

Chronic immune activation associated with chronic helminthic and human immunodeficiency virus infections: role of hyporesponsiveness and anergy

Chronic immune activation is one of the hallmarks of human immunodeficiency virus (HIV) infection. It is present also, with very similar characteristics, in very large human populations infested with helminthic infections. We have tried to review the studies addressing the changes in the immune profiles and responses of hosts infected with either one of these two chronic infections. Not surprisingly, several of the immune derangements and impairments seen in HIV infection, and considered by many to be the "specific" effects of HIV, can be found in helminth-infected but HIV-noninfected individuals and can thus be accounted for by the chronic immune activation itself. A less appreciated element in chronic immune activation is the immune suppression and anergy which it may generate. Both HIV and helminth infections represent this aspect in a very wide and illustrative way. Different degrees of anergy and immune hyporesponsiveness are present in these infections and probably have far-reaching effects on the ability of the host to cope with these and other infections. Furthermore, they may have important practical implications, especially with regard to protective vaccinations against AIDS, for populations chronically infected with helminths and therefore widely anergic. The current knowledge of the mechanisms responsible for the generation of anergy by chronic immune activation is thoroughly reviewed.

Characterization of Nef-CXCR4 interactions important for apoptosis induction

The HIV-1 Nef protein was analyzed for apoptotic structural motifs that interact with the CXCR4 receptor and induce apoptosis in CD4(+) lymphocytes. Two apoptotic motifs were identified. One centered on Nef amino acids (aa) 50 to 60, with the overlapping 20-mer peptides retaining about 82% of the activity of the full Nef protein. The second centered on aa 170 to 180, with the overlapping 20-mer peptides retaining about 30% of the activity of the full protein. Significant apoptotic abilities were observed for 11-mer motif peptides spanning aa 50 to 60 and aa 170 to 180, with a scrambled version of the 11-mer motif peptide corresponding to aa 50 to 60 showing no apoptotic ability. Hallmarks of apoptosis, such as the formation of DNA ladders and caspase activation, that were observed with the full-length protein were equally evident upon exposure of cells to these motif peptides. A CXCR4 antibody and the endogenous ligand SDF-1alpha were effective in blocking Nef peptide-induced apoptosis as well as the physical binding of a fluorescently tagged Nef protein, while CCR5 antibodies were ineffective. The CXCR4-negative cell line MDA-MB-468 was resistant to the apoptotic peptides and became sensitive to the apoptotic peptides upon transfection with a CXCR4-expressing vector. A fluorescently tagged motif peptide and Nef protein displayed physical binding to CXCR4-transfected MDA-MB-468 cells, but not to CCR5-transfected cells. The removal of the apoptotic motif sequences from the full-length protein completely eliminated the ability of Nef to induce apoptosis. However, these modified Nef proteins still retained the ability to enhance viral infectivity. Thus, specific sequences in the Nef protein appear to be necessary for Nef protein-induced apoptosis as well as for physical interaction with CXCR4 receptors.

B7+CTLA4+ T cells engage in T-T cell interactions that mediate apoptosis: a model for lentivirus-induced T cell depletion

Apoptosis in lymph node (LN) T cells of feline immunodeficiency virus (FIV)-infected cats is associated with cells co-expressing B7.1 and B7.2 costimulatory molecules, and their ligand CTLA4. To study the possibility of B7.1/B7.2-CTLA4 mediated T-T interactions and the predicted induction of T cell apoptosis in vitro, costimulatory molecules were up-regulated on CD4+ and CD8+ T cells by mitogen stimulation. B7.1 expression on in vitro stimulated CD4+ and CD8+ cells increased within 24h; B7.2 and CTLA4 expression increased after 48-72 h. Apoptosis, as analyzed by terminal deoxynucleotidyl transferase (transferase nick end labeling, TUNEL)-based staining followed by three color flow cytometric analysis, correlated to the cells expressing B7 and/or CTLA4. Blocking experiments revealed that CD4+ and CD8+ T cell apoptosis could be significantly inhibited with anti-B7 antibodies. As FIV infection results in immune activation with a T cell phenotype similar to that of the in vitro activated T cells, the data support the hypothesis that the chronic expansion of B7+CTLA4+ LN T cells in infected cats allows for T-T cell interactions resulting in T cell depletion and eventually the development of AIDS.

Extracellular Nef protein targets CD4+ T cells for apoptosis by interacting with CXCR4 surface receptors

The effects of soluble Nef protein on CD4(+) T cells were examined. CD4(+)-T-cell cultures exposed to soluble Nef were analyzed for apoptosis by terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling and hallmarks of apoptosis including cytoplasmic shrinkage, nuclear fragmentation, DNA laddering, and caspase activation. We observed dose- and time-dependent inductions of apoptosis. DNA laddering and activated caspase 3 were also evident. Cells treated with Nef/protein kinase inhibitor complexes were protected from Nef-induced apoptosis, suggesting possible roles for protein kinases in the apoptosis pathway. Similarly, cells treated with Nef/anti-Nef antibody complexes were protected from Nef-induced apoptosis. The cellular receptor responsible for Nef-induced apoptosis was identified through antibody- and ligand-blocking experiments as a receptor commonly involved in viral entry. CXCR4 antibodies, as well as the endogenous ligand SDF-1alpha, were effective in blocking Nef-induced apoptosis, while CCR5 and CD4 antibodies were ineffective. Moreover, a CXCR4-deficient cell line, MDA-MB-468, which was resistant to Nef-induced apoptosis, became sensitive upon transfection with a CXCR4-expressing vector. This study suggests that extracellular Nef protein could contribute to the decline of CD4 counts prior to and during the onset of AIDS in patients with human immunodeficiency virus type 1 infections.

Feline Immunodeficiency Virus Infection Phenotypically and Functionally Activates Immunosuppressive CD4+CD25+T Regulatory Cells

Disease progression of feline immunodeficiency virus (FIV) infection is characterized by up-regulation of B7.1 and B7.2 costimulatory molecules and their ligand CTLA4 on CD4(+) and CD8(+) T cells. The CD4(+)CTLA4(+)B7(+) phenotype described in FIV(+) cats is reminiscent of CD4(+)CD25(+)CTLA4(+) cells, a phenotype described for immunosuppressive T regulatory (Treg) cells. In the present study, we describe the phenotypic and functional characteristics of CD4(+)CD25(+) T cells in PBMC and lymph nodes (LN) of FIV(+) and control cats. Similar to Treg cells, feline CD4(+)CD25(+) but not CD4(+)CD25(-) T cells directly isolated from LN of FIV(+) cats do not produce IL-2 and fail to proliferate in response to mitogen stimulation. Unstimulated CD4(+)CD25(+) T cells from FIV(+) cats significantly suppress the proliferative response and the IL-2 production of Con A-stimulated autologous CD4(+)CD25(-) T cells compared with unstimulated CD4(+)CD25(+) T cells from FIV(-) cats. Flow-cytometric analysis confirmed the apparent activation phenotype of the CD4(+)CD25(+) cells in LN of chronically FIV(+) cats, because these cells showed significant up-regulation of expression of costimulatory molecules B7.1, B7.2, and CTLA4. These FIV-activated, anergic, immunosuppressive CD25(+)CTLA4(+)B7(+)CD4(+) Treg-like cells may contribute to the progressive loss of T cell immune function that is characteristic of FIV infection.

Subpopulations of long-lived and short-lived T cells in advanced HIV-1 infection

Antigenic stimulation of T cells gives rise to short-lived effector cells and long-lived memory cells. We used two stable isotope-labeling techniques to identify kinetically distinct subpopulations of T cells and to determine the effect of advanced infection with HIV-1. Long-term deuterated water (2H2O) incorporation into DNA demonstrated biphasic accrual of total and of memory/effector (m/e)-phenotype but not naive-phenotype T cells, consistent with the presence of short-lived and longer-lived subpopulations within the m/e-phenotype T cell pool. These results were mirrored by biphasic die-away kinetics in m/e- but not naive-phenotype T cells after short-term 2H-glucose labeling. Persistent label retention was observed in a subset of m/e-phenotype T cells (presumably memory T cells), confirming the presence of T cells with very different life spans in humans. In advanced HIV-1 infection, much higher proportions of T cells were short-lived, compared to healthy controls. Effective long-term anti-retroviral therapy restored values to normal. These results provide the first quantitative evidence that long-lived and quiescent T cells do indeed predominate in the T cell pool in humans and determine T cell pool size, as in rodents. The greatest impact of advanced HIV-1 infection is to reduce the generation of long-lived, potential progenitor T cells.

T cell dynamics in HIV-1 infection

In the absence of antiretroviral treatment, HIV-1 establishes a chronic, progressive infection of the human immune system that invariably, over the course of years, leads to its destruction and fatal immunodeficiency. Paradoxically, while viral replication is extensive throughout the course of infection, deterioration of conventional measures of immunity is slow, including the characteristic loss of CD4(+) T cells that is thought to play a key role in the development of immunodeficiency. This conundrum suggests that CD4(+) T cell-directed viral cytopathicity alone cannot explain the course of disease. Indeed, recent advances now indicate that HIV-1 pathogenesis is likely to result from a complex interplay between the virus and the immune system, particularly the mechanisms responsible for T cell homeostasis and regeneration. We review these data and present a model of HIV-1 pathogenesis in which the protracted loss of CD4(+) T cells results from early viral destruction of selected memory T cell populations, followed by a combination of profound increases in overall memory T cell turnover, damage to the thymus and other lymphoid tissues, and physiological limitations in peripheral CD4(+) T cell renewal.

Turnover rates of B cells, T cells, and NK cells in simian immunodeficiency virus-infected and uninfected rhesus macaques

We determined average cellular turnover rates by fitting mathematical models to 5-bromo-2'-deoxyuridine measurements in SIV-infected and uninfected rhesus macaques. The daily turnover rates of CD4(+) T cells, CD4(-) T cells, CD20(+) B cells, and CD16(+) NK cells in normal uninfected rhesus macaques were 1, 1, 2, and 2%, respectively. Daily turnover rates of CD45RA(-) memory T cells were 1%, and those of CD45RA(+) naive T cells were 0.5% for CD4(+) T cells and approximately 1% for CD4(-)CD45RA(+) T cells. In SIV-infected monkeys with high viral loads, the turnover rates of T cells were increased approximately 2-fold, and that of memory T cells approximately 3-fold. The turnover of CD4(+)CD45RA(+) naive T cells was increased 2-fold, whereas that of CD4(-)CD45RA(+) naive T cells was marginally increased. B cells and NK cells also had increased turnover in SIV-infected macaques, averaging 3 and 2.5% per day, respectively. For all cell types studied here the daily turnover rate increased with the decrease of the CD4 count that accompanied SIV infection. As a consequence, the turnover rates of CD4(+) T cells, CD4(-) T cells, B cells, and NK cells within each monkey are strongly correlated. This suggests that the cellular turnover of different lymphocyte populations is governed by a similar process which one could summarize as "generalized immune activation." Because the viral load and the CD4 T cell count are negatively correlated we cannot determine which of the two plays the most important role in this generalized immune activation.

Peripheral blood cytotoxic gammadelta T lymphocytes from patients with human immunodeficiency virus type 1 infection and AIDS lyse uninfected CD4+ T cells, and their cytocidal potential correlates with viral load

Progression of human immunodeficiency virus type 1 (HIV-1) infection in humans is marked by declining CD4+-T-cell counts and increasing virus load (VL). Cytotoxic T lymphocytes (CTL) play an important role in the lysis of HIV-infected cells, especially during the early phase of asymptomatic infection. CTL responses in the later phase of disease progression may not be as effective since progressors with lower CD4+-T-cell counts have consistently higher VL despite having elevated CTL counts. We hypothesized that, apart from antiviral effects, some CTL might also contribute to AIDS pathogenesis by depleting CD4+ T cells and that this CTL activity may correlate with the VL in AIDS patients. Therefore, a cross-sectional study of 31 HIV-1-infected patients at various clinical stages was carried out. Purified CTL from these donors as well as HIV-seronegative controls were used as effectors against different human cell targets by using standard 51Cr release cytolytic assays. A direct correlation between VL and CTL-mediated, major histocompatibility complex (MHC)-unrestricted lysis of primary CD4+-T-cell, CEM.NKR, and K562 targets was observed. CD4+-T-cell counts and duration of infection also correlated with MHC-unrestricted cytolytic activity. Our data clearly show that gammadelta CTL are abnormally expanded in the peripheral blood of HIV-infected patients and that the Vdelta1 subset of gammadelta T cells is the main effector population responsible for this type of cytolysis. The present data suggest that gammadelta CTL can contribute to the depletion of bystander CD4+ T cells in HIV-infected patients as a parallel mechanism to HIV-associated immunopathogenesis and hence expedite AIDS progression.

CD4 T cell depletion is linked directly to immune activation in the pathogenesis of HIV-1 and HIV-2 but only indirectly to the viral load

The causal relationships among CD4 cell depletion, HIV replication, and immune activation are not well understood. HIV-2 infection, "nature's experiment" with inherently attenuated HIV disease, provides additional insights into this issue. We report the finding that in HIV-2 and HIV-1 patients with a comparable degree of CD4 depletion the imbalance in the relative sizes of the naive and memory T cell populations and the up-regulation of CD4 and CD8 cell activation markers (HLA-DR, CD38, CD69, Fas molecules) are similar, even though the viral load in the plasma of HIV-2-infected patients is two orders of magnitude lower than in HIV-1 patients and HIV-2 patients are known to have slower rates of CD4 T cell decline and a better clinical prognosis. Moreover, we found a similar increase in the frequency of cycling CD4 T cells (Ki67+), which was in strong correlation with the expression of activation markers. Finally, the level of T cell anergy, as assessed by the proliferative responses to CD3 stimulation and to a panel of microbial Ags, proved to be comparable in HIV-1 and HIV-2 patients with a similar degree of CD4 depletion despite large differences in viral load. Our data are consistent with a direct causal relationship between immune activation and CD4 cell depletion in HIV disease and an only indirect relation of these parameters to the virus replication rate. Invoking the concept of proximal immune activation and virus transmission, which links efficient transmission of virus to local cell activation and proliferation in response to Ags and inflammation, we propose an integrative interpretation of the data and suggest that strongly elevated immune activation induces CD4 cell depletion and not vice versa, with potential implications for the choice of treatment strategies.

CTLA-4 upregulation during HIV infection: association with anergy and possible target for therapeutic intervention

OBJECTIVE

To study the role of cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4) during HIV infection.

METHODS

Intracellular CTLA-4 expression, determined by flow-cytometry, and proliferative responses to HIV antigens, were studied in peripheral blood mononuclear cells (PBMC) from 93 HIV-1-infected [HIV(+)] patients and 40 HIV-1 seronegative controls.

RESULTS

The proportions of CTLA-4 expressing CD4+ T cells were: (1) significantly higher in HIV(+) patients, 10.95 +/- 0.66%, than in controls, 6 +/- 0.45% (P < 0.0001); (2) inversely correlated to CD4+ counts (r = -0.67, P < 0.005, n = 16, drug-naive patients; r = -0.57, P < 0.0001, n = 77, HAART-treated patients); and (3) positively correlated to proportion of activated (HLA-DR+CD3+) (r = 0.53, P < 0.0001) and memory (CD45RO+CD4+) T cells (r = 0.46, P < 0.001). CD28 median fluorescence intensity in CTLA-4- cells was twice that in CTLA-4+ cells (140 +/- 5.3 versus 70 +/- 2.28, P < 0.00001), whereas cells low in CD28 and CD4, expressed more CTLA-4 (P < 0.0001). Higher proportion of CTLA-4+CD4+ cells expressed CCR5 and Ki-67, in comparison with CTLA-4-CD4+ cells, (65 +/- 11.9 and 25 +/- 7.5% versus 27 +/- 8.9 and 3.7 +/- 2%, P < 0.0001 and P < 0.01, respectively). Among HAART-treated patients, with viral load below detectable levels, CD4+ cells increase was inversely correlated to %CTLA-4+CD4+ cells (r = -0.5, P = 0.003, n = 39). Proliferation of PBMC to anti-CD3, gp-120 depleted HIV-1 antigen or HIV-1 p24 stimulation was inversely correlated with CTLA-4 levels (r = -0.68, P = 0.0035; r = -0.38,P = 0.04; and r = -0.43, P = 0.028, respectively).

CONCLUSIONS

(1) CTLA-4 is upregulated during HIV infection and may therefore account for CD4 T-cell decline and anergy in HIV-1 infection. (2) Increased levels of CTLA-4 may undermine immune responses and in the HAART-treated patient-immune reconstitution. (3) Blocking of CTLA-4 may offer a novel approach for immune-based therapy in HIV infection.

Contribution of immune activation to the pathogenesis and transmission of human immunodeficiency virus type 1 infection

The life cycle of human immunodeficiency virus type 1 (HIV-1) is intricately related to the activation state of the host cells supporting viral replication. Although cellular activation is essential to mount an effective host immune response to invading pathogens, paradoxically the marked systemic immune activation that accompanies HIV-1 infection in vivo may play an important role in sustaining phenomenal rates of HIV-1 replication in infected persons. Moreover, by inducing CD4+ cell loss by apoptosis, immune activation may further be central to the increased rate of CD4+ cell turnover and eventual development of CD4+ lymphocytopenia. In addition to HIV-1-induced immune activation, exogenous immune stimuli such as opportunistic infections may further impact the rate of HIV-1 replication systemically or at localized anatomical sites. Such stimuli may also lead to genotypic and phenotypic changes in the virus pool. Together, these various immunological effects on the biology of HIV-1 may potentially enhance disease progression in HIV-infected persons and may ultimately outweigh the beneficial aspects of antiviral immune responses. This may be particularly important for those living in developing countries, where there is little or no access to antiretroviral drugs and where frequent exposure to pathogenic organisms sustains a chronically heightened state of immune activation. Moreover, immune activation associated with sexually transmitted diseases, chorioamnionitis, and mastitis may have important local effects on HIV-1 replication that may increase the risk of sexual or mother-to-child transmission of HIV-1. The aim of this paper is to provide a broad review of the interrelationship between immune activation and the immunopathogenesis, transmission, progression, and treatment of HIV-1 infection in vivo.

Immune activation correlates better than HIV plasma viral load with CD4 T-cell decline during HIV infection

This study addressed the role of T-cell immune activation in determining HIV-1 plasma viral load and CD4+ T-cell blood levels during HIV-1 infection. A decrease of blood CD4 levels and CD4/CD8 ratios and an increase of CD8 levels in both treated (n = 35) and untreated (n = 19) HIV-positive individuals were more strongly correlated to immune activation (log percentage of HLA-DR+CD3+ cells; R = -0.78, R = -0.77, and R = 0.58, respectively; p <.0001) than to CD4 T-cell proliferation (log percentage of Ki-67+CD4+ cells; R = -0.57 [p <.0001], R = -0.48 [p <.001], and R = 0.37 [p <.01], respectively) or to viral load (R = -0.36 [p <.01], R = -0.23 [p =.09], R = 0.13 [p =.35], respectively). Because almost half of the Ki-67+CD4+ cells were also positive for CTLA-4 (a marker for activated nonproliferating cells), the correlation of CD4 levels to Ki-67 expression is only partially related to cell proliferation and more likely represents mainly immune activation of the cells without proliferation. Taken together, these results suggest that immune activation is the major determinant of CD4 decline and should therefore be considered central for the monitoring of HIV infection and its outcome after antiviral treatment.

The dynamics of CD4+ T-cell depletion in HIV disease

The size and composition of the CD4+ T-cell population is regulated by balanced proliferation of progenitor cells and death of mature progeny. After infection with the human immunodeficiency virus, this homeostasis is often disturbed and CD4+ T cells are instead depleted. Such depletion cannot result simply from accelerated destruction of mature CD4+ T cells - sources of T-cell production must also fail. Ironically, this failure may be precipitated by physiological mechanisms designed to maintain homeostasis in the face of accelerated T-cell loss.

Use of human CD4 transgenic mice for studying immunogenicity of HIV-1 envelope protein gp120

HIV-1 envelope protein, gp120, is a major immunogenic protein of the AIDS virus. A specific feature of this protein is its interaction with the receptor protein, human CD4, an important component of the immune system. This interaction might affect the immunogenic properties of the gp 120 and modulate the immune response towards HIV. To test this hypothesis we used human CD4-transgenic mice for immunization with gp120. The dynamics of the immune response towards gp120, CD4 and other proteins was followed. The results show that the primary immune response to gp120 (two weeks) developed somewhat faster in CD4-transgenic mice versus non-transgenic mice. Both animals, however, ultimately mounted the same level of response over time. The primary immune response to gp120 when complexed with soluble CD4 before the immunization, developed similarly in both groups. The secondary immune response was earlier and markedly stronger in non-transgenic mice compared with the transgenic mice where a less efficient memory response to gp120 was observed. The ability of gp120 to directly interact with CD4+ helper lymphocytes appears to affect the humoral response towards this antigen. Moreover, these effects illustrate how viral modulation of these cells may in turn lead to potentially different states of immunological equilibrium.

Rapid restoration of CD4 T cell subsets in subjects receiving antiretroviral therapy during primary HIV-1 infection

OBJECTIVE

To compare the effect of highly active antiretroviral therapy on immune reconstitution in subjects with acute and chronic HIV-1 infection.

DESIGN

Prospective study including 58 treatment-naive subjects who commenced indinavir or nelfinavir and two nucleosides during primary (PHI; n = 28) or chronic HIV-1 infection (CHI; n = 30).

METHODS

Naive (CD45RA+ 62L+), memory (CD45RA-) and activated (CD38+ HLA-DR+) T cell subsets were quantified at 1-2 monthly time intervals using 4-colour flow cytometry.

RESULTS

At 1 year, HIV-1 RNA declined in both cohorts to undetectable levels (< 50 copies/ml), while median CD4 lymphocyte count increased from 470 to 758 x 10(6) cells/l in PHI and from 204 to 310 x 10(6) cells/l in CHI, reaching > 500 x 10(6) cells/l in 93% of PHI, but only in 37% of CHI subjects (P < 0.001). Naive CD4 lymphocytes increased from 106 to 176 x 10(6) cells/l in PHI and from 41 to 44 x 10(6) cells/l in CHI (PHI versus CHI at 12 months: P = 0.003), while memory cells rose from 368 to 573 x 10(6) cells/l in PHI and from 148 to 223 x 10(6) cells/l in CHI (P < 0.001). Early increases (< 3 months) of CD4 lymphocytes were larger in subjects with PHI, consisting of naive CD45RA+ CD62L+ as well as memory CD45RA- CD62L+ cells (P = 0.001). CD4 activation declined from 5 to 2% in PHI and from 13 to 6% in CHI (P = 0.001), while CD8 cell activation was reduced from 33 to 15% in PHI and from 42 to 19% in CHI (P = 0.02).

CONCLUSION

Immune reconstitution was more complete, occurred earlier and comprised both naive and memory CD4 T lymphocytes in subjects who commenced antiretroviral therapy during primary HIV-1 infection.

Inverse correlation of telomerase activity/proliferation of CD4+ T lymphocytes and disease progression in simian immunodeficiency virus-infected nonhuman primates

Both increased lymphocyte renewal with subsequent exhaustion of the immune system and impaired T-cell renewal have been put forth to account for CD4+ T-cell depletion and development of AIDS in HIV-1-infected humans and SIV-infected nonhuman primates. In the present study, telomeric terminal restriction fragment length and telomerase activity were used as measures of proliferative activity of T lymphocytes from three nonhuman primate species before and after being infected with SIV. In peripheral blood T cells, our data show both species and T-cell-subset-specific differences in proliferative activity accompanied by different patterns of disease progression. A significant postinfection increase in telomerase/proliferative activity in CD4+ T cells from seropositive sooty mangabeys and from normal progressor rhesus macaques was associated with asymptomatic infection or delayed disease progression, respectively, whereas a decrease in telomerase/proliferative activity detected in CD4+ T cells postinfection from SIVsmmPBj14-infected pigtailed macaques was associated with rapid CD4+ T-cell depletion and disease progression. The levels of telomerase activity observed in CD4+ T cells from peripheral blood closely parallelled those seen in CD4+ T cells in lymph node samples from selected animals. Our data suggest that an increase in proliferative activity of T lymphocytes in vivo may be associated with a favorable course of SIV infection in nonhuman primates.

Dissociation of immunologic and virologic responses to highly active antiretroviral therapy

OBJECTIVE

Immunologic markers, levels of HIV DNA, and infectious HIV were compared in partial responders (PR) to HAART who had high plasma HIV RNA levels but stable or increasing levels of CD4+ peripheral blood mononuclear cells (PBMC), and patients with complete failure (CF) who had very low or decreasing levels of CD4+ PBMC and high plasma HIV RNA levels.

DESIGN AND METHODS

CD4 and CD8 levels were monitored by flow cytometry. Beta2-microglobulin (beta2M) and neopterin levels were measured by quantitative enzyme immunoassays. Plasma and PBMC from 11 PR and 13 CF were analyzed for infectious HIV levels in limiting dilution cultures. Polymerase chain reaction (PCR) assays were used to quantify cellular HIV DNA and plasma HIV RNA.

RESULTS

In comparison with CF, PR had little or no CD4+ cell loss, a substantial increase in CD8+ cells, significantly fewer positive plasma HIV cultures (p = .03), lower frequencies of infectious HIV in total PBMC (p = .005) and in CD4+ PBMC (p < .001), and lower frequencies of HIV DNA in CD4+ PBMC (p = .007).

CONCLUSIONS

Lower levels of infectious HIV and a lower frequency of CD4+ PBMC that contain "productive" HIV DNA in PR as compared with CF may contribute to the stable or increasing CD4+ PBMC levels of the PR. However, HAART may also have effects on lymphocyte homeostasis independent of its antiviral activity.

Could Nef and Vpr proteins contribute to disease progression by promoting depletion of bystander cells and prolonged survival of HIV-infected cells?

A growing body of literature suggests that the HIV accessory proteins Nef and Vpr could be involved in depletion of CD4(+) and non-CD4(+) cells and tissue atrophy, and in delaying the death of HIV-infected cells. Cell depletion is likely to be predominantly a bystander effect because the number of cells dying far outnumbers HIV-infected cells and is not confined to CD4(+) cells. The myristylated N-terminal region of Nef has severe membrane disordering properties, and when present in the extracellular medium causes rapid lysis in vitro of a wide range of CD4(+) and non-CD4(+) cells, suggesting a role for extracellular Nef in the depletion of bystander cells. A direct role for HIV-1 Nef in cytopathicity is supported by studies in HIV-infected Hu Liv/Thy SCID mice, in transgenic mice expressing nef gene alone, and in rhesus macaques infected with SIV/HIV chimeric virus containing HIV-1 nef. The N-terminal region of Nef has been directly implicated in development of simian AIDS. Extracellular Vpr and C-terminal fragments of Vpr cause membrane permeabilization and apoptosis of a wide range of CD4(+) and non-CD4(+) cells, and could also contribute to depletion of bystander cells. A direct in vivo role for Vpr in thymocyte depletion, thymic atrophy, and nephropathy is suggested in studies with vpr transgenic mice. Intracellular Nef and Vpr could help HIV-infected cells evade cell death by inhibiting apoptosis of infected cells and by avoiding virus-specific CTL response. Nef and Vpr are potential targets for therapeutic intervention and vaccine development, and strategies that prevent the death of bystander cells while promoting the early death of HIV-infected cells could arrest or retard progression to AIDS.